1. Field of the Invention
The present invention relates to a method for producing an organic light emitting device, such as organic light emitting diodes (OLEDs), a multi-color display and the like, having a multilayered structure with a substrate layer providing a first electrode layer, a second electrode layer, and at least one layer of light emitting organic material between the first and second electrode layers. Further, the invention is directed to a structure of an organic light emitting diode.
2. Description of the Related Art
Organic light emitting diodes (OLEDs) are an emerging technology with potential applications as discrete light emitting devices, or as the active element of light emitting arrays or displays, such as flat-panel displays. OLEDs are devices in which a stack of organic layers is sandwiched between two electrodes. At least one of the electrodes must be transparent in order for light, which is generated in the active region of the organic stack, to escape.
OLEDs emit light which is generated by organic electroluminescence (EL). Organic EL at low efficiency was observed many years ago in metal/organic/metal structures as, for example, reported in Pope et al., Journal Chem. Phys., Vol. 38, 1963, pp. 2024, and in xe2x80x9cRecombination Radiation in Anthracene Crystalsxe2x80x9d, Helfrich et al., Physical Review Letters, Vol. 14, No. 7, 1965, pp. 229-231.
Recent developments have been spurred largely by two reports of high efficiency organic EL. The two reports include C. W. Tang et al., xe2x80x9cOrganic electroluminescent diodesxe2x80x9d, Applied Physics Letters, Vol. 51, No. 12, 1987, pp. 913-915, and Burroughes et al., xe2x80x9cLight-emitting diodes based on conjugated polymersxe2x80x9d, Nature, Vol. 347, 1990, pp. 539-541. Tang et al. made two-layer organic light emitting devices using vacuum deposited molecular dye compounds, while Burroughes used spin coated poly(p-phenylenevinylene) (PPV), a polymer.
A standard organic light emitting diode (OLED) consists of a glass substrate covered with a transparent electrode (usually indium-tin oxide ITO) and one or more organic thin layers and a metal top contact. This type of diode is described in U.S. Pat. No. 5,247,190 (Friend et al.), U.S. Pat. No. 4,356,429 (Tang et al.) and in the above-mentioned articles of Tang et al. and Burroughes et al.
Using this type of OLEDs for display applications requires an external driving circuit chip and complex wirings and bondings. These problems are not present, if the organic display is integrated on its own driving circuit which provides the bottom electrodes for each pixel. Therefore, the electrode layer deposited on a substrate is structured in a manner having a multitude of single metal pads each defining an electrode for a pixel of the display. Attempts to realize such surface emitting diode structures have been described for example in Parker et al., Appl. Phys. Lett., Vol. 64, No. 14, 1994, pp. 1774-1176 or Baigent et al., Appl. Phys. Lett., Vol. 65, No. 21, 1994, pp. 2636-2638.
The organic layers are deposited on top of the structured electrode, and the display is completed with a common top electrode. In this case, a transparent top electrode is necessary, because generally the substrate with the driving circuit is opaque. However, the deposition of such a transparent lateral conductor (TLC) causes problems, especially for large area organic light emitting displays for the following reasons.
First, today the best known material for a transparent lateral conductor is the before mentioned indium-tin oxide (ITO). This material is generally sputter deposited. However, this relatively aggressive sputter process damages the underlying organic materials or even shorts to the bottom electrode.
Secondly, commonly-used transparent lateral conductors are metal oxides. During evaporation of suitable metals under an oxygen-rich atmosphere the organic materials are likely to degrade.
Thirdly, alternative suitable materials for transparent lateral conductors and processes which can be deposited very gently and can exhibit high transmittance and high conductivity are not known so far.
Fourthly, a thin semi-transparent metal electrode (TME) could be used for small areas of such kinds of large area displays. But for large areas, this semi-transparent metal electrode is not conductive enough to supply the high current needed to illuminate all pixels. For the pixels in the middle of the display, a tremendous voltage drop will occur in the thin metal electrode and higher voltages have to be applied to reach same brightness. However, making the semi-transparent metal electrode thicker would result in unacceptably high absorption of the light and consequently lead to a significantly reduced luminous inefficiency of the device.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a structure (and method) which allows the fabrication of devices without damaging the light emitting areas, but while providing a high lateral conductivity and a high transparency.
It is another object of the present invention to provide a new and improved method for production of an organic light emitting device, array and display based thereon by avoiding the before mentioned disadvantages.
It is yet another object of the present invention to provide organic light emitting devices having a large light emitting area of uniform brightness, a long service life, and easy and low-cost manufacturing of the same.
In a first aspect of the present invention, a structure and method is provided for producing an organic light emitting device having a multilayered structure with a substrate providing a first electrode layer, a second electrode layer and at least one layer of light emitting organic material between said first and second electrode layers. The second electrode layer includes at least two separate layers including a layer of a semi-transparent metal electrode (TME) and a layer of transparent lateral conductor (TLC). The layer of a semi-transparent metal electrode is deposited onto the layer of light emitting organic material. At least one light transparent layer being not light emitting (e.g., opaque) is deposited on the semi-transparent metal electrode (TME), to provide protection against the effects of a following deposition of the layer of light transparent conductive material (the transparent lateral conductor (TLC)).
The protection layer shields the light emitting organic material layer and the semi-transparent metal electrode (TME) from damage caused by the deposition process such as sputtering or evaporation of ITO under oxygen atmosphere.
The protection layer may be formed of generally transparent insulators or semiconductors such as GaN, AlN, SiN or other Nitrides, or CaF, MgF or other Fluorides, or photoresists or epoxy resins. Besides using indium-tin oxide (ITO) for the transparent lateral conductor (TLC), InGaN, InO or other transparent and conductive metal oxides are also applicable.
In a second aspect of the invention, based on the underlying inventive idea of providing a protection layer, the present invention provides a method for producing a surface emitting, large area organic multi-color display.
In this aspect, at the surface of a substrate, the first layer of electrode is deposited in a structured manner, as described above, for rendering a multitude of electrode pads defining the areas of light emitting pixels of the device. A stack of layers of organic material is deposited upon the first electrode covering both the electrode pads as well as the non-covered regions of the substrate.
Onto the upper surface of the organic material a layer of conductive material is applied such as a thin layer of a semi-transparent metal electrode (TME). This layer acts as the top carrier injection contact to the organic layers and has to supply only the current for one single pixel.
On top of the semi-transparent metal electrode, a first protection layer made of GaN, AlN, SiN or other nitrides or CaF, MgF or other fluorides, is deposited homogeneously and covers the whole surface. On top of this first protection layer and the semi-transparent metal electrode, a second protection layer in a patterned manner is deposited. The second protection layer, which is made of a photoresist material or epoxy resin and therefore suitable for photolithography or other methods of patterning, is patterned such that the light emitting areas, which are each defined by the areas of each electrode pad in the first electrode layer, of the display (pixels) are covered with the material. The space in-between the pixels is only covered by the first protection layer.
In the subsequent sputtering or evaporation step for depositing, the transparent lateral conductor made of, for example, ITO, the lighting areas (e.g., pixel areas) are covered and protected by the first and second protection layers. The areas between the transparent lateral conductors must form a contact with the semi-transparent metal electrode. Thus, the thin layer of the first protection layer is made either conductive or the deposition process of the transparent lateral conductor penetrates the first protection layer such that a contact with the semi-transparent metal electrode occurs.
With the inventive method, a highly, transparent common top electrode can be deposited without damaging the areas of the active organic layers. The electrical contact from the transparent lateral conductor to the lighting pixel area is made via the space between the pixels where the TLC is in contact with the semi-transparent metal electrode. With this process, large area displays can be fabricated, because the semi-transparent metal electrode is highly transparent and sufficiently thick enough to supply enough current for one pixel. The high current density occurs in the thick transparent lateral conductor which supplies current for the entire display.